The detection and/or measurement of particles in a fluid has become increasingly important over the past few years, and devices and methods have heretofore been suggested and/or utilized for achieving such detection and/or measurement.
More recently, particle detectors utilizing lasers have been suggested for use in detection and/or measurement, including particle sizing (see, for example, U.S. Pat. No. 3,406,289 and the following articles: R. G. Knollenberg, "An Active Scattering Aerosol Spectrometer", Atmospheric Technology, Number 2, June, 1973, pages 80-81; R. G. Knollenberg, "Active Scattering Aerosol Spectrometry", Dental Bureau of Standards Special Publication 412, issued October, 1974, pages 57-64; R. G. Knollenberg and R. E. Leuhr, "Open Cavity Laser `Active` Scattering Particle Spectrometry From 0.05 to 5 Microns", Fine Particles, Aerosol, Generation Measurement, Sampling and Analysis, Editor Benjamin Y. H. Liu, Academic Press, May, 1975, pages 669-696; R. G. Knollenberg, "Three New Instruments For Cloud Physics Measurement: The 2-D Spectrometer, the Forward Scattering Spectrometer Probe, and the Active Scattering Aerosol Spectrometer", American Meterological Society, International Conference on Cloud Physics, July, 1976, pages 554-561; and R. G. Knollenberg, "The Use of Low Power Lasers in Particle Size Spectrometry", Proceedings of the Society of Photo-Optical Instrumentation Engineers: Practical Applications of Low Power Lasers, Volume 92, August, 1976, pages 137-152.
The detection and/or measurement of particles suspended in a liquid media is of tremendous importance, for example, to the semiconductor and related electronic component industries since such particles have been found to constitute a level of microcontamination sufficient to reduce manufacturing yields to an unprofitable level. Typical process liquids used in these industries include deionized water, photoresists, strong acids and bases, hydrocarbon solvents and proprietary mixtures of chemicals.
Instruments that are currently used to monitor contaminates within such process liquids are largely optical using light scattering to size particles, and these instruments can be categorized as belonging to one of two possible classes--"in-situ" instruments that measure remotely and sample a small portion of the total fluid volume utilized, and "volumetric" instruments that sample all of the fluid volume utilized (a discussion of in-situ and volumetric instruments can be found in "`In situ` Optical Particle Size Measurements in Liquid Media", by Robert G. Knollenberg, Proceedings of Pure Water Conference, Palo Alto, Calif. Jan. 13-14, 1983).
Both the in-situ and volumetric instruments have characteristics which allow them to be optimally used under different circumstances. Both types of these presently known instruments, however, can logically view only a small illuminated volume if maximum sensitivity is desired (as is required for microcontamination measurements, for example).
A volumetric instrument must have a highly restricted passage to allow all of the fluid to pass through the illuminated view or monitoring region. Typical dimensions of such a passage to provide the necessary restriction are on the order of one millimeter diameter to thereby enable maximum fluid flows of 100 to 200 ml/min.
An in-situ instrument, on the other hand, while having no required flow restrictions (since only a small portion of the fluid is sampled), is nevertheless presently restricted in viewing volume by established optical parameters (field-of-view, depth-of-field, etc.) rather than by physical boundaries.
Heretofore, in-situ instruments have exhibited superior performance characteristics as compared with known volumetric instruments. One reason for this is that known volumetric instruments have required interfaces between the fluid and the fluid confining vessel walls and such interfaces are sources of large amounts of stray light. This stray light establishes a noise background level from which light scattered by individual particles must be differentiated. Obviously, if the noise background is greater than the particle scattering signal, the particles within the fluid cannot be detected or measured. With in-situ type instruments, the particles are illuminated and viewed through windows whose fluid interfaces can be removed far enough away from the illuminated view-volume (monitoring region) to be out of the depth-of-field and the light scattering noise contribution has therefore been of a negligible nature.
Thus, known in-situ and volumetric instruments, while having been found to be useful, nevertheless have not been found to be fully satisfactory, and further improvements therein would therefore be found useful.